This application is based upon and claims priority from Korean Patent Application No. 10-2001-76802 filed Dec. 6, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a process for preparing xcex2-D-ribofuranose derivatives, which are useful as an intermediate for the preparation of ribavirin with an antiviral activity.
2. Description of the Related Art
xcex2-D-Ribofuranose derivatives are useful as an intermediate for the preparation of ribavirin. Processes for the preparation thereof are disclosed in U.S. Pat. No. 3,798,209 and J. Med. Chem., 1972, Vol. 15, No.11, 1150xcx9c1154. The process disclosed in U.S. Pat. No. 3,798,209 may be summarized in the following reaction scheme 1: 
In the above reaction scheme 1, Bz is a benzoyl group.
However, it takes about 3 days to complete the reaction of the above process. Further, additional isolation processes are necessary because the compounds of formula (Ixe2x80x3) are also produced as a by-product. In order to carry out the isolation processes, re-crystallization and/or column chromatography are conventionally performed, which complicates an industrial-scale mass production.
The process disclosed in J. Med. Chem., 1972, Vol. 15, No. 11, 1150xcx9c1154 is illustrated in the following reaction scheme 2: 
In the above reaction scheme 2, Ac is an acetyl group; and R1 is a protecting group for hydroxyl group.
The reaction, as shown in the scheme 2, is carried out at a high temperature, such as 160xcx9c165xc2x0 C., which causes a degradation of compounds including reactants and products. Degraded compounds are changed into tar, which makes it difficult to isolate and purify an end product. The yield of the end product through the above process is relatively low, i.e., about 74xcx9c78%. There is needed an additional distillation equipment under a reduced pressure, in order to remove acetic acid that is generated during the reaction.
Further, N,O-bis(p-nitrophenyl) phosphate, used as a catalyst, is very expensive reagent, which is unfavorable for industrial-scale mass production of xcex2-D-ribofuranose derivatives. Moreover, since the above process gives the compound of formula (Ixe2x80x2xe2x80x3) with gray color (i.e., low purity), an additional purification process is required in order to produce a high purity of ribavirin.
The present invention provides a process for preparing xcex2-D-ribofuranose derivatives in high purity and yield under a mild condition, so as to be favorably applied to a large-scale mass production thereof.
In one aspect of the present invention, there is provided a process for preparing a compound of formula (I), which comprises (a) reacting a compound of formula (II) with a compound of formula (III) at 80xc2x0 C.xcx9c120xc2x0 C., in the presence of a catalyst selected from the group consisting of methanesulfonic acid, benzenesulfonic acid and p-toluenesulfonic acid; and (b) crystallizing the resulting compound obtained in the step (a): 
wherein, R1 is a protecting group for hydroxyl group; and R2 is a C1-C4 alkyl group.
In one embodiment of the present invention, a compound of formula (II) and a compound of formula (III) are reacted under a mild condition in the presence of a catalyst and a resulting compound is crystallized, in accordance with the following reaction scheme 3 described below: 
In the above reaction scheme 3, R1 is a protecting group for hydroxyl group; and R2 is a C1-C4 alkyl group.
The compounds of formula (II) and (III), which are starting materials in the process of the present invention, may be prepared by a method which is known in the art (e.g., J. Med. Chem., 1972, Vol. 15, No. 11, 1150xcx9c1154).
The protecting group for hydroxyl group includes any conventional protecting groups which are acceptable for the preparation of antiviral agents, such as acetyl group, benzoyl group and etc.
The eq. ratio of the compound of formula (II): the compound of formula (III) is preferably 1:1.0xcx9c1.2. Where eq. ratio of the compound of formula (III) to the compound of formula (II) is less than 1, the compound of formula (II) may remain un-reacted. Where the compound of formula (III) is used more than 1.2 eq. to 1 eq. of the compound of formula (II), the color of the product, i.e., the compound of formula (I), may be changed (yellow). Further, un-reacted compound of formula (III), which could remain in the reaction system, may be converted to 1,2,4-triazole-3-carboxamide during a subsequent ammonolysis for the preparation of ribavirin, which then remains as one of impurities.
In reacting the compounds of formula (II) and (III), a higher temperature raises the rate of the reaction. However, considering purity and yield of the product, the reaction is preferably performed at 80xc2x0 C.xcx9c120xc2x0 C., more preferably 90xc2x0 C.xcx9c100xc2x0 C.
The catalyst is selected from the group consisting of methanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid. These catalytic compounds are much cheaper than N,O-bis(p-nitrophenyl) phosphate. In addition, the catalyst allows the reaction to be carried out under a milder condition. Thus, a high reaction temperature can be avoided. Among those catalytic compounds, p-toluenesulfonic acid is preferable, in the aspect of the yield of the product.
The catalyst may be added to the reaction mixture preferably at a ratio of 0.01xcx9c0.05 eq. based on 1 eq. of the compound of formula (II), considering the yield of the product.
The reaction of the compounds of formula (II) and (III) does not require a solvent. That is, both compounds of formula (II) and (III) melt at the reaction temperature (i.e., 80xc2x0 C.xcx9c120xc2x0 C.) and the reaction proceeds by adding the catalyst. The reaction may be completed within preferably 3xcx9c6 hours, more preferably 4xcx9c5 hours.
In crystallizing the resulting compound, a solvent is used. The solvent for crystallization includes an alcohol, such as methanol, ethanol, butanol, isobutanol, and an aqueous solution thereof. Among them, more preferable solvent for crystallization is methanol, considering a purity of the product, a compound of formula (I).
The compound of formula (I) obtained according to the present invention can be converted to ribavirin in accordance with conventional methods disclosed in, e.g., U.S. Pat. No. 3,798,209 and J. Med. Chem., 1972, Vol. 15, No. 11, 1150xcx9c1154. For example, through ammonolysis and re-crystallization, high purity ribavirin of white color can be easily obtained from the compound of formula (I) without any additional purification and/or decolorization.
The present invention is further illustrated and described by the following examples, which should not be taken to limit the scope of the invention.